ATLANTA — Mayor Bloomberg yesterday urged more people to donate charitably during tough times, in an address on philanthropy.

In his keynote address to the Council on Foundations annual conference, the billionaire mayor recalled learning about philanthropy when, as a boy, his father gave a $50 donation to the NAACP.

Bloomberg also outlined how charitable giving has helped the city, such as relief efforts after Sept. 11, tree planting and a program that gives cash payments to poor people for expenses that have long-term benefits, like medical care and job training.

“I’m in a business where revenues go down when the needs go up,” he told the conference in Atlanta. “The most potent force is private philanthropy and the private sector, and the government.”